1. Field of the Invention
The field of the invention is percussion hammer drills, which are accelerated downward by the force of gravity, and which carry an internal hammer to strike additional blows shortly after an attached drill bit has first contacted the bottom of a hole.
2. Description of the Prior Art.
One type of percussion drilling is performed by cable drilling rigs with a drill string that is suspended from a steel cable to be lifted and then quickly released. A relatively heavy drill stem has an upper end coupled to the cable through a socket. The drill stem carries a steel or carbide-tipped drill bit at its lower end.
In my previous U.S. Pat. Nos. 3,215,212; 3,409,091 and 3,409,095, I have provided an internal hammer in the drill stem that acts against springs or other biasing means to strike a second blow shortly after the bit has first contacted the bottom of the hole being drilled. Whereas the first impact of the bit cuts into the rock or earth formation being drilled, the second impact increases the fragmentation of the material. Usually in drills of this type successive blows are caused by the rebounding of the internal hammer to produce a chattering effect.
While the prior art drill structures have performed reasonably well, there is need for drill stems with larger internal hammers. To be more specific, hammers in the form of a long cylindrical rod with a diameter of up to eighteen inches are desired. The hammer has an anvil at its lower end which strikes an anvil within the casing, and it is desirable to have anvil contact surfaces with the largest area possible in order to distribute the forces of impact and lower the stress in these elements.
The hammer in the gravity drill of U.S. Pat. No. 3,409,091 had two portions of reduced diameter for carrying a pair of springs, while the hammers in U.S. Pat. Nos. 3,409,095 and 3,215,212 had more than one portion of reduced diameter for other reasons that are apparent from their disclosures. For the large drills currently being contemplated, it would be advantageous to be able to provide a hammer of a large mass that fills an internal bore of the drill stem to the maximum extent possible, and require only a single such reduced portion.
Because the hammer is enclosed within the drill stem chamber and is subjected to strong impacts, a certain amount of metallic dust and debris accumulates in the closed interior. Such dust and debris can accumulate and clog the springs near the lower end of the drills of the prior art, thereby limiting the life of these tools. The prior tools were not constructed with a view to periodic maintenance, and maintenance required the inconvenient step of cutting open the casing. There was thus a need for a drill with longer life and better serviceability.